It is common in the motion pictures industry to optically duplicate the camera negative after it has been processed onto a positive intermediate film. The positive intermediate film is processed and then copied onto one or more negative intermediate films. The negative intermediate film is then processed and contact printed onto print film which is processed and sent to theaters to be projected. This final process of contact printing has to be done at very high speed since many copies are needed for distribution to many theaters within a short period of time.
A contact printer, shown schematically in FIG. 1, can print up to 2,000 feet per minute. The print film 12, the copy film, is placed on sprocket wheel 37 and the negative intermediate film 11, the master film, is placed on top of the print film on the same sprocket wheel 37. Light source 39 exposes the print film 12 through the negative intermediate film 11. Because of the thickness of the film stock, the two films, which are wrapped one on top of the other around the same sprocket, are driven at slightly different operating radii. This difference in transport speed is addressed by using a shorter perforation pitch for the print film. But even with this solution there is the possibility a build-up of tension between the films which may cause a sudden film displacement, and create a visible image shift on screen when the print film is projected.
There are additional problems associated with the contact printer described above. If the films are not held in intimate contact, image loss may occur due to defocusing of the image. If, however, the two films are held at high pressure Newton rings can occur, or in extreme cases one film may physically damage the other, resulting in loss of registration and decrease in image quality of the print film. Yet another severe problem with contact dry printers is the possibility of dirt transfer from the print film onto the intermediate negative film which diminishes the quality of subsequent prints from the same intermediate film.
Optical printing is another way to duplicate film. In this process the negative intermediate film and the print film are not in contact but instead a lens is used to image one film onto the other. The films are moved on mechanical shuttles, one frame at a time, and are stationary during exposure and image registration, thus the position of the frame in respect to the perforations is good. However, due to the start and stop motion of the film transport system, this duplication method is slow and typically operates at less than 200 feet per minute.
A Cosharp printer, described by John Mosely in the SMPTE Journal, Sep. 1992, was an attempt to combine the speed of the continuous web transport of the contact printer and the attributes of optical printing. This printer used a common transport shaft on which the negative intermediate 65 mm films, and the 35 mm print film were placed. In this device the copy registration is not affected by speed variations of the transport. An optical system provides demagnification from the 65 mm format to the 35 mm format. Since the two formats used different perforation systems, i.e. 5 perforation per frame in one and 4 perforations per frame in the other, the mechanical design accommodates this difference by using different diameter sprockets on a common transport shaft. While the Cosharp system did provide for the placement of both the intermediate negative and the print film on the same transport shaft, it used a very complex optical system incorporating 28 spherical elements, 4 cylinder elements and two large flat mirrors.
An important image quality issue in film duplication is matte and scratches. Matte is the term used to describe a layer of transparent beads with diameters of a couple of microns which are spread on one or both sides of a film to control friction and prevent film sticking when spooled. Matte surfaces are basically phase objects, which can be rendered less visible as the angular spread of the illumination is increased. When the film is illuminated with specular light, i.e. light with limited angular spread, the phase objects become visible in a manner similar to the way a phase contrast microscope transforms phase variations in a transparent object into an amplitude variations. Scratches in the emulsion or base are also phase objects. When the negative intermediate film is coated with matte or is scratched, and is specularly illuminated, the image may be degraded and the scratches and matte may become visible on the copy.
The suppression of matte and scratches is common in the photofinishing industry where 35 mm film images are commonly printed onto photographic paper using diffuse illumination. Kessler, U.S. Pat. No. 5,221,975 discusses scratch and matte suppression by diffuse illumination in the context of a film CCD scanner. However, in the motion pictures industry, most of the optical printing today is done with relatively specular illumination, i.e. with the angular extent of the light at the film plane being under.+-.6.degree.. For good scratch and matte suppression much larger angles are needed, on the order of.+-.30.degree. or above. Thus current optical film duplicators do not suppress matte and scratches well. When there is a need for scratch or matte suppression, the current solution is to use a contact printer or an optical printer equipped with a wet gate. Such printers are slow, however, on the order of up to 200 feet per minute, due to restrictions on the film speed in the liquid, and the need to dry the solution from the film prior to spooling.
Another problem with the optical printers used in the film industry today is that they use lenses, such as the Printing Nikkor lens, which contain multiple refractive elements. These lenses, though of relatively good quality, are limited in terms of the veiling glare, due to a large number of glass components. Low flare, low veiling glare, and high resolution optics for 1:1 imaging is disclosed by Offner in U.S. Pat. No. 4,467,361, wherein two mirrors provide for unit magnification imaging optics for the microlithographic industry. Kessler in U.S. Pat. No. 5,221,975 discloses the use of the Offner system for film scanners. The attributes of this optical system in terms of cost and ease of alignment were also discussed by Kessler. However, the reflective, monocentric imagers disclosed by Offner and Kessler are not adapted for use in film printers in the motion pictures industry.